HARDNESS is the property of material which refers to the resistance to indentation. Hardness is also refer to the stiffness to the resistance against scratching, cutting, abrasion. If more is the hardness the greater is the resistance to the indentation.
Hardness can be given in different units like BHN, HRC, HRB, HV. All the units are for hardness but they are used for different hardness value.

BHN: BHN is an abbreviation for Brinell hardness Number. Brinell hardness is found by pressing a hard steel or carbide sphere of a standard diameter under a specified load onto the surface of a material. Then measuring the diameter of the indentation left after the test.The Brinell hardness number is calculated by dividing the applied force in kgf by surface area of the impression in mm2 

HRC/HRB: HRC/HRB is an abbreviation for Rockwell hardness number. Rockwell Hardness test is a hardness measurement based on the net increase in depth of impression as a load is applied. Hardness value have no number but are commonly given the alphabate in the B, C, K, L, M, P, R, S, V. The Rockwell hardness testerand Brinell hardness Tester are almost same but differ in a way that depth of  impression is measured in Rockweel Hardness tester where as Diameter of Impression is measured in Brinell Hardness tester. The ASTM (American Society for Testing & Materials) has standardized a set of scales (ranges) for Rockwell hardness testing. Each scale is designated by a letter.

A: Cemented carbides, thin steel and shallow case hardened steel
B: Copper alloys, soft steels, aluminum alloys, malleable iron, etc.
C: Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel and other
      Materials harder than B 100
D: Thin steel and medium case hardened steel and pearlitic malleable iron
E: Cast iron, aluminum and magnesium alloys, bearing metals
F: Annealed copper alloys, thin soft sheet metals
G: Phosphor bronze, beryllium copper, malleable irons
H: Aluminum, zinc, lead

K, L, M, P, R, S, V: Bearing metals and other very soft or thin materials, including plastics.  

HV: HV is an abbreviation Vickers Hardness Number. This method of Hardness measurement is used to find hardness of extremely hard surface. The surface of material is subjected to a standard pressure for a standard length of time by means of a pyramid-shaped diamond. In this test diagonal of the final imression on the material is measured under a microscope and the Vickers Hardness value read from a conversion table. The Vickers test is reliable for measuring the hardness of metals, and also used on ceramic materials.

Apart from above mentioned hardness test Knoop hardness test and Shore hardness test are also used to find hardness values.

For Hardness conversion follow the below link: